I completely disagree that changing the length of rests will screw over some classes. Nothing but changing the frequency of encounters between rests. And that's entirely up to the DM.
Also the ratio between types of rests is a factor.
As we all know, D&D has always depended on pacing to impose balance upon class resources and encounter challenge levels. In the olden days it was just spells/day classes vs unlimited sword-swinging classes vs encounters/day. 5e added short rests and a few classes that primarily used them. And it has resource- rich, moderate, and poor classes, of both sorts, so there are broadly 6 types of character class resource schedules in 5e to be balanced against eachother & encounters (and other challenges), vs two back in the day.
Resource-rich classes have /lots/ of abilities, generally spells, and lots of their overall effectiveness rests in those abilities, but with a lower at-will baseline. Examples of daily resource-rich classes are all the full-casters, except for the Warlock, which is the sole resource-rich short-rest class.
Moderate-resource classes still have a fair variety of recharge abilities, but much of their power remains in their at-will baseline performance. Half-casters, and 1/3rd-caster sub-classes are examples of daily-recharge moderate-resource classes. The Monk is the main example of the short-rest version.
Resource poor classes base their performance mainly on their at-will baseline, they might have one fairly potent limited-use ability, or a few minor ones that still don't provide much versatility. The Barbarian stands as the example of resource poor with a daily resource, while the fighter is the prime example of a short-rest recharge.
A 2:1 short:long rest ratio balances uses/day of high-resource classes like full casters on a daily schedule with the Warlock; 1/2 & 1/3 casters with the Monk; and the low-daily-resouce Barbarian with the low-short-rest-resouce Fighter. It doesn't make much difference if there are one or several encounters between those short rests when balancing classes at the same resource level, but different recharge rates.
Imposing balance /among/ those three resource levels, however, depends on the higher-resource classes being forced to fall back on their at-will baseline often enough for the lower-resouce classes' higher baseline to make up the difference.
By the same token, with all classes putting forth about the same overall performance, at that point, a day's worth of encounters will be about as challenging as intended.
So it's more complex than just pegging encounter balance by getting in your 6-8 encounters. You also need to get a short:long rest ratio just right. And, combats need to be long enough (in rounds) to see everyone falling back on at-wills at least some of the time.
Even given all the above done just right, there's still a tendency for the classes with the most potency concentrated in their limited-use abilities to shine the brightest /when it matters the most/, as they bring the most powerful abilities in the most critical situations, while others' "time to shine" is in filler combats and the trailing rounds of 'mop up,' after an encounter has been decided.
Changing the timing of rests while keeping the frequency of encounters between them allows the DM to fit it to a more relaxed narrative than the dungeon crawl, say a several week trek across a savanna, without always giving full rests between encounters and giving a big bonus to the long-rest-recovery classes.
True: 5e's prescribed 6-8 medium/hard encounter 2-3 short rest "day" can be changed to a week (or whatever) to pace a whole campaign more slowly, but not /differently/, because the factors 5e needs you to impose balance on are more complex than ever.
IMHO, it makes even more sense for the DM to impose restrictions on and vary the time it takes to rest, in order to fit the expected ratio of encoutners:short:long rests to not just of the campaign overall, but of the specific story at any give point in the campaign.
Edit: Finally, I guess it's worth noting that there are just a few odd-class out short-rest 'troublemakers.' That is, you can cut just a few classes and simplify everything tremendously. If you cut the Warlock, Monk, and Fighter, you'll've eliminated just about all the meaningful short rest resources except HD. The Barbarian, Paladin, and Ranger should be able to handle all your thwacking and twanging needs.